This invention relates to reduction of color distortion due to the effects of noise on gamma correctors.
Gamma correctors are used in television systems to correct for the nonlinearity of signal-to-light transducers such as kinescopes or light-to-signal transducers such as camera tubes or vidicons. The gamma corrector generally takes the form of a load resistor network for a transistor which network is interconnected with diodes to provide a plurality of gain break points at particular voltage values. A gain/voltage characteristic is thereby generated which is selected to compensate for the nonlinearities of the transducer. The video signal is ordinarily closely controlled before being applied to the gamma corrector, because the absolute value of the signal establishes the instantaneous gain of the gamma amplifier stage. Thus, it is common in a camera to clamp the video signal to ground during the blanking intervals, so that the black level is always at the origin of the gamma curve. Grey and white-going portions of the signal are then more positive than ground, and are translated through the gamma corrector with the correct curvature or nonlinearity for compensation of the nonlinearity of the transducers. The curvature of the gamma corrector may also correct for such other effects as defects in the color motion-picture film in a telecine converter, and the like.
It has been observed that color errors result from noise superimposed on the black portions of the television signal. The color error results from rectification of noise near black signal level. The rectification may result from the properties of the gamma corrector, which has no response (no output signal) for signal excursions into the blacker-than-black region resulting from noise, or the rectification may result from an actual black-level clipper preceding the matrix. This in turn causes the signal attributable to each of the R, G or B signals to be superimposed on a varying component created by rectification of the noise. Even if the R, G and B tubes in a three-tube color camera had exactly the same noise color error would still occur on an ordinary colored image because the rectified noise component of each signal represents a different proportion of the signal in that channel, and the color of the image displayed from composite color signals depends upon their relative proportions. For a white image, color error would occur even if the three R, G and B tubes produced the same amount of noise, if the sensitivities of the tubes were different.
It might be thought that the black level of the signal could be moved up on a standard gamma curve so that the peak negative-going noise component would not drop into the region in which the gamma corrector gives no response, but this has been found not to be sufficient to correct the color distortion resulting from noise near black level.